> For the complete documentation index, see [llms.txt](https://todepond.gitbook.io/spacetode/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://todepond.gitbook.io/spacetode/inputs.md).

# Inputs

Inputs are the symbols you write on the left-hand-side of a rule diagram to check an atom's surroundings.\
In the SandPond engine, there are some in-built inputs that you can use straight away:

* `@` The centre of the diagram.
* `_` An empty space.
* `#` A non-empty space.
* `.` Any space.
* `*` Not a space (ie: the edge of the universe).
* `$` This element.

You can also [make your own input symbols](/spacetode/symbols.md) by using the following keywords:

* `symbol`
* `given`
* `select`
* `check`